I agree with previous posters re checking Trip Advisor for reviews of hotels. Reviews for many of the timeshare resorts (e.g., Marriott Vacation Club, Hilton Vacation Club, Sheraton, etc.) as well as the Disney motels, hotels, and timeshare villas can be found on Trip Advisor. If you have questions about a specific property after reading the reviews on Trip Advisor, don't hesitate to the post the question here, as I'm sure some of us may have info about a particular property.

There is a recent thread rehashing pros/cons of onsite/offsite. Hopefully, you will find it on the first two pages of this board. This topic comes up occaisionally and if the search function is working you might find past threads re onsite vs. offsite.

Short version for pros of onsite:

Total immersion (aka Disney Magic) in Disney (you will find many vacationers who exclusively stay on site noting this as a primary reason and why the extra cost for Disney accomodations is worth the extra money. Said magic includes wake up callse from Mickey Mouse or other characters, depending on the level of accomodation you have (Disney offers the following price scales--value, moderate, Deluxe, and its own brand of timeshare--DVC--soaps and toiletries wit Mickey Mouse's picture on the item; housekeeping staff making animals and other figures out of the towels. Disney accomodations have great lanscaping and elaborate pools (at the moderate and deluxe price range). Deluxe hotels have froom service as well as full-service restaurants, cocktail lounges, pools bars, and fast food and coffee bar options. Moderate accomodations have limited room service (pizza, I believe), aforementioned bars and table service dining. Value accomodations have food courts.
Reduces or eliminates the need for a rental car if you're staying exclusively on Disney property because transfer service at the airport (Magical Express) takes you (and your luggage) to your WDW accomodation. There are buses, water taxis, and the monorail to get you from your room to the theme park, water park, a different WDW hotel/motel, or the shopping and entertainment of Downtown Disney.

Proximity to theme parks--this is true only if you are staying in the most expensive accomodation level (the deluxe hotels). From the deluxe accomodations near Epcot and MGM, you can actually walk to these parks. You can walk to the Magic Kingdom from the Contemporary.

Early Magic Hours--a perk only for guests staying onsite allowing them the opportunity to visit a designated park in the morning or evening during special hours.

Park accessibility--during peak periods (e.g., Christmas) the theme parks get really crowded. When capacity is exceeded, entrance to the parks is limited to onsite guests only.

Dining Reservations--I'm not 100% sure on this but I think onsite guests get a wider window for making reservations at the various table service restaurants in the theme parks or in the hotels. Depending on the time of year you trave--and the restaurant--this could be a needed perk. Remember, a dining reservation at Disney is not the same as a reservation elsewhere in that you call a restaurant and request seating for specific time, show up at that time and you are seated. Reservations Disney-style mean that you will be seated as soon as possible within the timeframe of your reservations.

If all or some of the above are important to you, then onsite accomodations may be what you might want to choose. Onsite might be a better option if this is your first visit to WDW as the entire complex (four theme parks, two water parks, shopping/entertainment complex (aka Downtown Disney), golf courses, miniature golf facilities, and numerous hotels/motels as well as a campground. I find WDW rather easy to navigate, but some people do not and prefer to leave the driving to Disney.

Pros for Offsite
Accomodations with larger rooms and more amenities than those offered by Disney (e.g., free breakfast--from full hot breakfast to basic continental--free Wifi for Internet access, etc. Some offsite properties are nicely landscaped and have fancy pools) for significantly lower prices. If you are interested in deluxe level or condo/villa accomodations, offsite simply offers better value. Many offsite properties offer transportation to the WDW parks but the service is limited to specific drop off/pick up times, butI consider car rental a necessity when staying offsite (I consider a necessity when staying onsite as well, as I don't like waiting for buses).

Other considerations--offsite guests can make reservations (ADRs) for WDW restaurants. Offsite guests may use the various onsite transportation options. EMH is quite popular and those parks with allocated time for onsite guests get really crowded. Regardles if you stay onsite or offsite, avoid the EMH park.

I've always just looked at the basic travel web-site, Expedia and Travelocity. Both of them will give you lots of good info and pictures. And don't forget to check out the hotels in DownTown Disney. They have their own web-site too.